1. Field of Invention
This invention relates to document scanners and converting their analog video signals to binary video signals.
2. Background of Prior Art
In the field of document scanning and image processing, the output of linear array scanners are analog video signals. These analog video signals represent continuously varying shades of gray. One objective is to convert these analog video signals to binary video signals by comparing a thresholding signal with the analog video signal on a pixel by pixel basis. The generation of the thresholding signal is critical if an accurate binary video representation of printed document is desired. Generally, an aperture or window is electrically synthesized from the primary video signal. Several scan lines are synchronously stored until the aperture is filled. The aperture makes available a matrix of pixels in the local vicinity of and including the pixel of interest. A function such as median[AR], weighted average[AR], or local peak/valley[AK] operates on the matrix of pixels and generates a threshold level for a given pixel. The next pixel of interest will have an aperture with a different matrix of pixels and so on. Most threshold functions are adaptive in that each threshold level is calculated from its aperture on a pixel by pixel basis. The aperture can be of one dimension(1D) or two dimension(2D) and any number of pixels in either dimension. When linear CCD's are used as scanners, one dimensional aperture synthesis generally requires some sort of delay lines or temporary storage of data and additional circuitry. When a two-dimensional aperture synthesis system is employed, several scans of data must be stored as well and then synchronously made available as required by the aperture. Complexity and quantity of electronic circuitry is proportional to any increase of the aperture in either dimension.